This invention relates to load-release devices of the kind whereby a load force or torque transmitted by the device is released when the load force or torque exceeds some preset value.
For this purpose, it is conventional to use shear pins each having one end closely fitted in one of the two members and the other end closely fitted in the other member. The shear pins shear when the load to be transmitted by them exceeds the maximum value determined by their shear strength.
Such shear pins have to be very accurately made if their performance is to be at all consistant and predictable. They have to be carefully fitted into finely finished bores in the two members. Whenever the pins shear in response to overload, the apparatus has to be dismantled, at least to some extent, and new pins carefully fitted by hand. This work must be carried out by a skilled fitter and is time-consuming and thus expensive not only in respect of the fitter's time but also in respect of the corresponding period of time during which the equipment driven through the two members is out of service. Furthermore, there may be a risk that fragments of the shear pins are ejected when they shear on overload. Any steps taken to minimise this risk, for example by the use of additional guards or housings entail further work and time in dismantling and reassembly each time the pins are replaced.
Finally, it is necessary to ensure that an adequate stock of the correct replacement shear pins is available at all times even though they may be very seldom required.